The present invention relates in general to semiconductor packaging and in particular to an apparatus and method of manufacture for a multi-chip high performance flip chip package for semiconductor devices.
While silicon process technology has advanced significantly in the past decade, for the most part, the same decades-old package technology continues as the primary packaging means. Epoxy or solder die attach along with aluminum or gold wire bonding to lead frame is still the preferred semiconductor packaging methodology. Advances in semiconductor processing technology, however, have made the parasitics associated with conventional packages more of a performance limiting factor. This is particularly true in the case of power switching devices where, as in the case of power MOSFETs, the on-resistance of these devices continues to push the lower limits. Thus, the parasitic resistance introduced by the bond wires and the lead frame in conventional packages becomes much more significant for such high current devices as power MOSFETs. Furthermore, the continuous shrinking of geometries and the resulting increase in chip densities has given rise to an increasing demand for semiconductor packages with lead counts higher than that offered by the conventional packaging techniques. given rise to an increasing demand for semiconductor packages with lead counts higher than that offered by the conventional packaging techniques.
Ball grid array and flip chip technologies were developed to address some of these demands. Both of these packaging technologies provide for a more direct connection between the silicon die and the printed circuit board as well as providing for higher interconnect densities. There is always room for improvement however. For example, a typical ball grid array package consists of a BT resin laminated board which serves as an interposer layer between the silicon die and the printed circuit board (PCB). Because of poor heat dissipation from the laminated board, external heat sinks and additional PCB copper layers are often required to dissipate excess heat.
In the case of conventional flip chip technology, among other shortcomings, heat dissipation is essentially governed by the die size and connection to the back side of the die is not easily facilitated (often requiring a bond wire connection). These limitationsxe2x80x94poor heat dissipation and resistive contact to back sidexe2x80x94become quite significant in high current applications such as power switching devices. A substantial improvement in the performance of flip chip packages is offered by Bencuya et al. in commonly assigned provisional Patent Application No. 60/088,651, filed Jun. 9, 1998, entitled xe2x80x9cLow Resistance Package for Semiconductor Devices.xe2x80x9d In one embodiment, this improved package eliminates wire bonding by making direct connection between an array of solder balls on one conductive surface of the die and a lead frame element, while connection to the opposite side is made by a die attach mechanism. This package exhibits significantly lower resistance; however, it still relies on a lead frame which adds residual resistance to the current path, and is not the smallest package possible for a given die size.
The desire to integrate increasing amounts of electronic circuitry into fewer separate components has led to multi-chip module (MCM) technology. The MCM technology allows two or more silicon chips to be mounted on a single carrier (or substrate) which is housed inside one package. The multiple chips on the common substrate can be interconnected using a variety of methodologies including face-up wire bonding, face-up tape automated bonding, and flip chip. The drawbacks associated with using flip chip interconnection in high current applications continue to persist in the MCM environment. The conventional MCM packaging introduces an additional problem in that the substrate upon which the multiple dies are mounted is common to all of the individual dies. An electrically common substrate severely limits the range of applications for the flip chip-connected MCM device. This is true, for example, in the case of power switching MOSFET devices discussed above. In these types of devices, the substrate or backside of each die acts as the drain terminal of the power MOSFET. An MCM carrier with an electrically common substrate, therefore, would not allow packaging of MOSFETs that require separate drain connections.
There is therefore a need for a high density semiconductor package whose attributes are minimal parasitic resistance and good heat dissipation, and that is readily manufacturable.
The present invention provides an improved multi-chip flip chip package that provides electrically isolated substrates, reduces package resistance to a negligible level, and offers superior thermal performance. Broadly, according to the present invention, a multi-chip carrier is provided that is made up of a first base layer separated from a second leadframe layer by a non-conductive laminating medium. The second leadframe layer is etched to form electrically isolated cavities to receive separate silicon dies. A silicon die is attached inside each cavity which is designed to surround the die along one or more edges of the die. Direct connection of the active surface of the silicon die to the printed circuit board is facilitated by an array of solder bumps that is distributed across the surface of each die as well as the edges of the leadframe layer surrounding each die. In one embodiment where connection to the back side of each die is required, the outer array of solder balls provide for low resistance electrical connection to the backside of each die that is attached to the leadframe layer. In applications where no connection to the back side of the die is required, the leadframe layer and the array of solder balls connecting to it may act as a thermal via for dissipating heat. Alternatively, the leadframe layer may be of dielectric material with selective conductive traces to make selective contact to traces on the board through the outer array of solder balls.
The multi-chip package of the present invention also reduces the number of steps required in the assembly process flow and is manufactured using standard materials and equipment. The resulting multi-chip package exhibits minimal resistance, improved heat dissipation, and is very thin and light as well as being cost-effective to manufacture. Furthermore, because of its construction, the multi-chip package of the present invention is able to withstand higher mechanical stress. It therefore enjoys improved reliability without the need for underfill that would otherwise be required for stress relief. Since each die would thus be essentially unencapsulated, the package may be rated at higher maximum junction temperatures, allowing for yet higher heat dissipation.
Accordingly, in one embodiment, the present invention provides A multi-chip semiconductor package including: a base layer; a dielectric layer of laminating medium disposed over the base layer; an electrically conductive lead frame layer disposed over the dielectric layer, the lead frame layer being physically divided into a plurality of electrically isolated lead frame sections, each section having a cavity; a plurality of silicon dies each having its substrate attached inside and making electrical contact with a respective cavity; and an inner array of solder balls distributed across an active surface of each of the plurality of silicon dies, and an outer array of solder balls disposed on a surface of cavity edges of the lead frame layers.
In another embodiment, the present invention provides a method for packaging a plurality of silicon dies inside a single package including the steps of: forming a multi-layer carrier having a base layer, a laminating dielectric middle layer and a conductive lead frame upper layer; etching through the lead frame layer to form a plurality of electrically isolated lead frame sections; stamping a cavity inside each of the plurality of electrically isolated lead frame sections; attaching a first surface of a silicon die inside each cavity such that a second surface of the silicon die and a surface of the edges of the lead frame layer adjacent to the silicon die form a substantially uniform plane; and disposing an array of solder balls across the substantially uniform plane with an outer array connecting to the lead frame layer and an inner array connecting to the second surface of the silicon die.